Pneumatic servomotors, such as disclosed in U.S. Pat. No. 3,106,873 are constructed in such a manner that air from the surrounding environment is directly presented to the rear chamber of the servomotor in response to an operator input force being applied to a control valve. As air enters the rear chamber, a differential pressure is created across a movable wall which supplies a master cylinder with an operational force sufficient to make a brake application. However, after a period of use, the operation of such servomotor can become unacceptable since airborne contaminates can adversely affect the operational control valve. Consequently, in-line air filters are required in the air supply conduit to prevent contaminants from being communicated to the control valve. Unfortunately, after extended periods of use, such in-line air filters can also become contaminated and the air flow therethrough reduced. As a result, such servomotors cannot meet the brake standardization test currently proposed by the United States Department of Transportation.